diff --git a/src/com/google/javascript/jscomp/CompilerInput.java b/src/com/google/javascript/jscomp/CompilerInput.java
index 97c394b4db3..c8754ca634f 100644
--- a/src/com/google/javascript/jscomp/CompilerInput.java
+++ b/src/com/google/javascript/jscomp/CompilerInput.java
@@ -37,7 +37,6 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
-import java.util.Set;
 import java.util.TreeMap;
 
 /**
@@ -499,7 +498,7 @@ public boolean isModule() {
     return "goog".equals(getLoadFlags().get("module"));
   }
 
-  private static <T> Set<T> concat(Iterable<T> first, Iterable<T> second) {
+  private static <T> ImmutableSet<T> concat(Iterable<T> first, Iterable<T> second) {
     return ImmutableSet.<T>builder().addAll(first).addAll(second).build();
   }
 
diff --git a/src/com/google/javascript/jscomp/CompilerOptions.java b/src/com/google/javascript/jscomp/CompilerOptions.java
index 82d01b4b76f..5b7c3c4e36f 100644
--- a/src/com/google/javascript/jscomp/CompilerOptions.java
+++ b/src/com/google/javascript/jscomp/CompilerOptions.java
@@ -1405,11 +1405,8 @@ public Map<String, Node> getTweakReplacements() {
     return getReplacementsHelper(tweakReplacements);
   }
 
-  /**
-   * Creates a map of String->Node from a map of String->Number/String/Boolean.
-   */
-  private static Map<String, Node> getReplacementsHelper(
-      Map<String, Object> source) {
+  /** Creates a map of String->Node from a map of String->Number/String/Boolean. */
+  private static ImmutableMap<String, Node> getReplacementsHelper(Map<String, Object> source) {
     ImmutableMap.Builder<String, Node> map = ImmutableMap.builder();
     for (Map.Entry<String, Object> entry : source.entrySet()) {
       String name = entry.getKey();
diff --git a/src/com/google/javascript/jscomp/EmptyMessageBundle.java b/src/com/google/javascript/jscomp/EmptyMessageBundle.java
index a03d433ba38..2715f7f3715 100644
--- a/src/com/google/javascript/jscomp/EmptyMessageBundle.java
+++ b/src/com/google/javascript/jscomp/EmptyMessageBundle.java
@@ -42,11 +42,9 @@ public JsMessage getMessage(String id) {
     return null;
   }
 
-  /**
-   * Returns an empty list of messages.
-   */
+  /** Returns an empty list of messages. */
   @Override
-  public Iterable<JsMessage> getAllMessages() {
+  public ImmutableList<JsMessage> getAllMessages() {
     return ImmutableList.of();
   }
 }
diff --git a/src/com/google/javascript/jscomp/JSModule.java b/src/com/google/javascript/jscomp/JSModule.java
index 32c93861e7c..e73a941b79a 100644
--- a/src/com/google/javascript/jscomp/JSModule.java
+++ b/src/com/google/javascript/jscomp/JSModule.java
@@ -81,12 +81,12 @@ public void setName(String name) {
   }
 
   @Override
-  public List<String> getProvides() {
+  public ImmutableList<String> getProvides() {
     return ImmutableList.of(name);
   }
 
   @Override
-  public List<String> getRequires() {
+  public ImmutableList<String> getRequires() {
     ImmutableList.Builder<String> builder = ImmutableList.builder();
     for (JSModule m : deps) {
       builder.add(m.getName());
diff --git a/src/com/google/javascript/jscomp/JSModuleGraph.java b/src/com/google/javascript/jscomp/JSModuleGraph.java
index 6e50d1d1df7..c10ba35c79d 100644
--- a/src/com/google/javascript/jscomp/JSModuleGraph.java
+++ b/src/com/google/javascript/jscomp/JSModuleGraph.java
@@ -426,19 +426,16 @@ public List<CompilerInput> manageDependencies(
   }
 
   /**
-   * Apply the dependency options to the list of sources, returning a new
-   * source list re-ordering and dropping files as necessary.
-   * This module graph will be updated to reflect the new list.
+   * Apply the dependency options to the list of sources, returning a new source list re-ordering
+   * and dropping files as necessary. This module graph will be updated to reflect the new list.
    *
-   * @param inputs The original list of sources. Used to ensure that the sort
-   *     is stable.
-   * @throws MissingProvideException if an entry point was not provided
-   *     by any of the inputs.
+   * @param inputs The original list of sources. Used to ensure that the sort is stable.
+   * @throws MissingProvideException if an entry point was not provided by any of the inputs.
    * @see DependencyOptions for more info on how this works.
    */
-  public List<CompilerInput> manageDependencies(
-      DependencyOptions depOptions,
-      List<CompilerInput> inputs) throws MissingProvideException, MissingModuleException {
+  public ImmutableList<CompilerInput> manageDependencies(
+      DependencyOptions depOptions, List<CompilerInput> inputs)
+      throws MissingProvideException, MissingModuleException {
 
     SortedDependencies<CompilerInput> sorter = new Es6SortedDependencies<>(inputs);
 
diff --git a/src/com/google/javascript/jscomp/NodeUtil.java b/src/com/google/javascript/jscomp/NodeUtil.java
index c3919b98e9e..e8e8133c39c 100644
--- a/src/com/google/javascript/jscomp/NodeUtil.java
+++ b/src/com/google/javascript/jscomp/NodeUtil.java
@@ -5468,7 +5468,7 @@ public static boolean isCallTo(Node n, String qualifiedName) {
     return n.isCall() && n.getFirstChild().matchesQualifiedName(qualifiedName);
   }
 
-  static Set<String> collectExternVariableNames(AbstractCompiler compiler, Node externs) {
+  static ImmutableSet<String> collectExternVariableNames(AbstractCompiler compiler, Node externs) {
     ReferenceCollectingCallback externsRefs =
         new ReferenceCollectingCallback(
             compiler,
diff --git a/src/com/google/javascript/jscomp/VariableMap.java b/src/com/google/javascript/jscomp/VariableMap.java
index 56bd6e96fae..07494264de3 100644
--- a/src/com/google/javascript/jscomp/VariableMap.java
+++ b/src/com/google/javascript/jscomp/VariableMap.java
@@ -26,7 +26,6 @@
 import com.google.common.collect.ImmutableSortedMap;
 import com.google.common.collect.ImmutableSortedSet;
 import com.google.common.io.Files;
-
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.IOException;
@@ -76,17 +75,13 @@ public String lookupSourceName(String newName) {
     return map.inverse().get(newName);
   }
 
-  /**
-   * Returns an unmodifiable mapping from original names to new names.
-   */
-  public Map<String, String> getOriginalNameToNewNameMap() {
+  /** Returns an unmodifiable mapping from original names to new names. */
+  public ImmutableMap<String, String> getOriginalNameToNewNameMap() {
     return ImmutableSortedMap.copyOf(map);
   }
 
-  /**
-   * Returns an unmodifiable mapping from new names to original names.
-   */
-  public Map<String, String> getNewNameToOriginalNameMap() {
+  /** Returns an unmodifiable mapping from new names to original names. */
+  public ImmutableMap<String, String> getNewNameToOriginalNameMap() {
     return map.inverse();
   }
 
@@ -206,7 +201,7 @@ public static VariableMap fromMap(Map<String, String> map) {
   }
 
   @VisibleForTesting
-  Map<String, String> toMap() {
+  ImmutableMap<String, String> toMap() {
     return map;
   }
 }
diff --git a/src/com/google/javascript/jscomp/newtypes/JSType.java b/src/com/google/javascript/jscomp/newtypes/JSType.java
index 5e611dc1897..ae5d98dd280 100644
--- a/src/com/google/javascript/jscomp/newtypes/JSType.java
+++ b/src/com/google/javascript/jscomp/newtypes/JSType.java
@@ -24,6 +24,7 @@
 import com.google.common.base.Function;
 import com.google.common.base.Joiner;
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableCollection;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
@@ -1874,7 +1875,7 @@ public final Node getSource() {
   }
 
   @Override
-  public final Collection<FunctionTypeI> getDirectSubTypes() {
+  public final ImmutableCollection<FunctionTypeI> getDirectSubTypes() {
     Preconditions.checkState(this.isConstructor() || this.isInterface());
     ImmutableList.Builder<FunctionTypeI> result = ImmutableList.builder();
     NominalType nt =
@@ -2105,7 +2106,7 @@ public final boolean hasProperty(String propertyName) {
   }
 
   @Override
-  public final Iterable<JSType> getUnionMembers() {
+  public final ImmutableCollection<JSType> getUnionMembers() {
     if (!isUnion()) {
       return ImmutableList.of(this);
     }
diff --git a/src/com/google/javascript/jscomp/newtypes/NominalType.java b/src/com/google/javascript/jscomp/newtypes/NominalType.java
index e41948f7e69..5e1a619ffab 100644
--- a/src/com/google/javascript/jscomp/newtypes/NominalType.java
+++ b/src/com/google/javascript/jscomp/newtypes/NominalType.java
@@ -98,7 +98,7 @@ ObjectKind getObjectKind() {
     return this.rawType.getObjectKind();
   }
 
-  Map<String, JSType> getTypeMap() {
+  ImmutableMap<String, JSType> getTypeMap() {
     return typeMap;
   }
 
diff --git a/src/com/google/javascript/jscomp/newtypes/TypeParameters.java b/src/com/google/javascript/jscomp/newtypes/TypeParameters.java
index 36edadfef4f..f18ce25e8b7 100644
--- a/src/com/google/javascript/jscomp/newtypes/TypeParameters.java
+++ b/src/com/google/javascript/jscomp/newtypes/TypeParameters.java
@@ -54,15 +54,15 @@ static TypeParameters make(List<String> ordinaryTypeParams, Map<String, Node> tt
     return new TypeParameters(builder.build());
   }
 
-  List<String> asList() {
+  ImmutableList<String> asList() {
     return this.typeParams.keySet().asList();
   }
 
   /**
-   * Returns a non-null list of type variables, by filtering out the TTL variables
-   * from this.typeParams.
+   * Returns a non-null list of type variables, by filtering out the TTL variables from
+   * this.typeParams.
    */
-  List<String> getOrdinaryTypeParams() {
+  ImmutableList<String> getOrdinaryTypeParams() {
     boolean foundTtlVariable = false;
     for (Map.Entry<String, Node> entry : this.typeParams.entrySet()) {
       if (!entry.getValue().isEmpty()) {
@@ -83,10 +83,10 @@ List<String> getOrdinaryTypeParams() {
   }
 
   /**
-   * Returns a non-null map from TTL variables to their ASTs, by filtering out the non-TTL
-   * variables from this.typeParams.
+   * Returns a non-null map from TTL variables to their ASTs, by filtering out the non-TTL variables
+   * from this.typeParams.
    */
-  public Map<String, Node> getTypeTransformations() {
+  public ImmutableMap<String, Node> getTypeTransformations() {
     boolean foundOrdinaryTypeVariable = false;
     for (Map.Entry<String, Node> entry : this.typeParams.entrySet()) {
       if (entry.getValue().isEmpty()) {