Add new FileAwareWarningsGuard that can give the script node for an e…

…rror

The @Suppress based WarningsGuard currently uses this functionality, and we'd
like to also this same functionality to the CheckIjsWarningsGuard

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185019159

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

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2018 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;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.javascript.rhino.Node;
+import javax.annotation.Nullable;
+
+/**
+ * An abstract WarningsGuard that provides an additional getScriptNodeForError() method
+ * for accessing the containing SCRIPT node of the AST in a robust way.
+ */
+public abstract class FileAwareWarningsGuard extends WarningsGuard {
+
+  private final AbstractCompiler compiler;
+
+  protected FileAwareWarningsGuard(AbstractCompiler compiler) {
+    this.compiler = compiler;
+  }
+
+  @Nullable
+  protected final Node getScriptNodeForError(JSError error) {
+    if (error.sourceName == null) {
+      return null;
+    }
+    Node scriptNode = compiler.getScriptNode(error.sourceName);
+    if (scriptNode != null) {
+      // TODO(b/73088845): This should always be a SCRIPT node
+      if (!scriptNode.isScript()) {
+        return null;
+      }
+      checkState(scriptNode.isScript(), scriptNode);
+      return scriptNode;
+    }
+    return null;
+  }
+}

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

@@ -31,19 +31,17 @@
  *
  * @author nicksantos@google.com (Nick Santos)
  */
-class SuppressDocWarningsGuard extends WarningsGuard {
+class SuppressDocWarningsGuard extends FileAwareWarningsGuard {
   private static final long serialVersionUID = 1L;
 
-  private final AbstractCompiler compiler;
-
   /** Warnings guards for each suppressible warnings group, indexed by name. */
   private final Map<String, DiagnosticGroupWarningsGuard> suppressors =
        new HashMap<>();
 
   /** The suppressible groups, indexed by name. */
   SuppressDocWarningsGuard(
       AbstractCompiler compiler, Map<String, DiagnosticGroup> suppressibleGroups) {
-    this.compiler = compiler;
+    super(compiler);
     for (Map.Entry<String, DiagnosticGroup> entry : suppressibleGroups.entrySet()) {
       suppressors.put(
           entry.getKey(),
@@ -84,8 +82,8 @@ class SuppressDocWarningsGuard extends WarningsGuard {
   @Override
   public CheckLevel level(JSError error) {
     Node node = error.node;
-    if (node == null && error.sourceName != null) {
-      node = compiler.getScriptNode(error.sourceName);
+    if (node == null) {
+      node = getScriptNodeForError(error);
     }
     if (node != null) {
       for (Node current = node;