Adds a system for sharing indexes (+builder) across multiple passes.

Following CLs will use this system to register incrementally updated
shared datastructures and the first will be for DefinitionUseSiteFinder.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=159857712

src/com/google/javascript/jscomp/AbstractCompiler.java

@@ -316,6 +316,15 @@ LifeCycleStage getLifeCycleStage() {
    */
   abstract void removeChangeHandler(CodeChangeHandler handler);
 
+  /** Register a provider for some type of index. */
+  abstract void addIndexProvider(IndexProvider<?> indexProvider);
+
+  /**
+   * Returns, from a provider, the desired index of type T, otherwise null if no provider is
+   * registered for the given type.
+   */
+  abstract <T> T getIndex(Class<T> type);
+
   /** Let the PhaseOptimizer know which scope a pass is currently analyzing */
   abstract void setChangeScope(Node n);
 

src/com/google/javascript/jscomp/Compiler.java

@@ -304,9 +304,9 @@ public Compiler() {
   /**
    * Creates a Compiler that reports errors and warnings to an output stream.
    */
-  public Compiler(PrintStream stream) {
+  public Compiler(PrintStream outStream) {
     addChangeHandler(recentChange);
-    this.outStream = stream;
+    this.outStream = outStream;
   }
 
   /**
@@ -2500,6 +2500,8 @@ void recordFunctionInformation() {
 
   protected final RecentChange recentChange = new RecentChange();
   private final List<CodeChangeHandler> codeChangeHandlers = new ArrayList<>();
+  private final Map<Class<?>, IndexProvider<?>> indexProvidersByType =
+      new LinkedHashMap<>();
 
   /** Name of the synthetic input that holds synthesized externs. */
   static final String SYNTHETIC_EXTERNS = "{SyntheticVarsDeclar}";
@@ -2525,6 +2527,25 @@ void removeChangeHandler(CodeChangeHandler handler) {
     codeChangeHandlers.remove(handler);
   }
 
+  @Override
+  void addIndexProvider(IndexProvider<?> indexProvider) {
+    Class<?> type = indexProvider.getType();
+    if (indexProvidersByType.put(type, indexProvider) != null) {
+      throw new IllegalStateException(
+          "A provider is already registered for index of type " + type.getSimpleName());
+    }
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  <T> T getIndex(Class<T> key) {
+    IndexProvider<T> indexProvider = (IndexProvider<T>) indexProvidersByType.get(key);
+    if (indexProvider == null) {
+      return null;
+    }
+    return indexProvider.get();
+  }
+
   Node getExternsRoot() {
     return externsRoot;
   }
@@ -2585,12 +2606,12 @@ private Node getChangeScopeForNode(Node n) {
       return n;
     }
 
-    n = NodeUtil.getEnclosingChangeScopeRoot(n.getParent());
-    if (n == null) {
+    Node enclosingScopeNode = NodeUtil.getEnclosingChangeScopeRoot(n.getParent());
+    if (enclosingScopeNode == null) {
       throw new IllegalStateException(
           "An enclosing scope is required for change reports but node " + n + " doesn't have one.");
     }
-    return n;
+    return enclosingScopeNode;
   }
 
   private void recordChange(Node n) {

src/com/google/javascript/jscomp/IndexProvider.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2017 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.javascript.jscomp;
+
+/** Builds a datastructure and incrementally updates it when scopes change. */
+interface IndexProvider<T> {
+
+  /** Returns an instance of type T. */
+  T get();
+
+  /** Returns a class literal specialized on T. */
+  Class<T> getType();
+}

test/com/google/javascript/jscomp/CompilerTest.java

@@ -1337,6 +1337,66 @@ public void testGetChangesAndDeletions_onlySeesChangesSinceLastRequest() {
     assertThat(compiler.getDeletedScopeNodesForPass("FunctionInliner")).isEmpty();
   }
 
+  public void testAddIndexProvider_ThenGetIndex() {
+    Compiler compiler = new Compiler();
+
+    compiler.addIndexProvider(new IndexProvider<String>() {
+      @Override
+      public String get() {
+        // Normally some shared index would be constructed/updated/returned here.
+        return "String";
+      }
+
+      @Override
+      public Class<String> getType() {
+        return String.class;
+      }});
+    compiler.addIndexProvider(new IndexProvider<Double>() {
+      @Override
+      public Double get() {
+        // Normally some shared index would be constructed/updated/returned here.
+        return Double.MAX_VALUE;
+      }
+
+      @Override
+      public Class<Double> getType() {
+        return Double.class;
+      }});
+
+    // Have registered providers.
+    assertThat(compiler.getIndex(String.class)).isEqualTo("String");
+    assertThat(compiler.getIndex(Double.class)).isEqualTo(Double.MAX_VALUE);
+
+    // Has no registered provider.
+    assertThat(compiler.getIndex(Object.class)).isNull();
+  }
+
+  public void testAddIndexProviderTwice_isException() {
+    Compiler compiler = new Compiler();
+
+    IndexProvider<String> stringIndexProvider =
+        new IndexProvider<String>() {
+          @Override
+          public String get() {
+            // Normally some shared index would be constructed/updated/returned here.
+            return "String";
+          }
+
+          @Override
+          public Class<String> getType() {
+            return String.class;
+          }
+        };
+    compiler.addIndexProvider(stringIndexProvider);
+
+    try {
+      compiler.addIndexProvider(stringIndexProvider);
+      fail("expected duplicate index addition to fail");
+    } catch (IllegalStateException e) {
+      // expected
+    }
+  }
+
   private static CompilerOptions createNewFlagBasedOptions() {
     CompilerOptions opt = new CompilerOptions();
     CompilationLevel.ADVANCED_OPTIMIZATIONS.setOptionsForCompilationLevel(opt);