/*
 * Copyright 2016 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.jscomp.NodeTraversal.AbstractPreOrderCallback;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;

/**
 * Checks to make sure the required ES6 externs are present.
 */
final class Es6ExternsCheck extends AbstractPreOrderCallback implements CompilerPass {
  static final DiagnosticType MISSING_ES6_EXTERNS =
      DiagnosticType.error("JSC_MISSING_ES6_EXTERNS",
          "Missing externs definition for Symbol. Did you forget to include the ES6 externs?");

  private final AbstractCompiler compiler;
  private boolean hasSymbolExterns = false;

  Es6ExternsCheck(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  private boolean hasEs6Syntax(Node root) {
    checkState(root.isRoot());
    for (Node script : root.children()) {
      checkState(script.isScript());
      if (TranspilationPasses.isScriptEs6OrHigher(script)) {
        FeatureSet features = (FeatureSet) script.getProp(Node.FEATURE_SET);
        if (!features.has(Feature.BLOCK_SCOPED_FUNCTION_DECLARATION)) {
          return true;
        } else {
          features = features.without(Feature.BLOCK_SCOPED_FUNCTION_DECLARATION);
          return !FeatureSet.ES5.contains(features);
        }
      }
    }
    return false;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseEs6(compiler, externs, this);
    if (!hasSymbolExterns) {
      if (hasEs6Syntax(root)) {
        compiler.report(JSError.make(MISSING_ES6_EXTERNS));
      }
      VarCheck.createSynthesizedExternVar(compiler, "Symbol");
    }
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    if (NodeUtil.isFunctionDeclaration(n) && n.getFirstChild().matchesQualifiedName("Symbol")) {
      hasSymbolExterns = true;
    }
    return !hasSymbolExterns;
  }
}

@@ -789,142 +789,8 @@
 ck;%0A
-import com.google.javascript.jscomp.parsing.parser.FeatureSet;%0Aimport com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;%0A
 impo
@@ -1583,333 +1583,20 @@
     
-FeatureSet features = (FeatureSet) script.getProp(Node.FEATURE_SET);%0A        if (!features.has(Feature.BLOCK_SCOPED_FUNCTION_DECLARATION)) %7B%0A          return true;%0A        %7D else %7B%0A          features = features.without(Feature.BLOCK_SCOPED_FUNCTION_DECLARATION);%0A          return !FeatureSet.ES5.contains(features);%0A        %7D
+return true;
 %0A   

Simplify Es6ExternsCheck to not special-case BLOCK_SCOPED_FUNCTION_DECLARATION.