Make UnreachableCodeElimination understand ES6 features.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158748650

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

@@ -2443,16 +2443,19 @@ protected CompilerPass create(AbstractCompiler compiler) {
     }
   };
 
-  /**
-   * Use data flow analysis to remove dead branches.
-   */
+  /** Use data flow analysis to remove dead branches. */
   private final PassFactory removeUnreachableCode =
       new PassFactory(Compiler.UNREACHABLE_CODE_ELIM_NAME, false) {
-    @Override
-    protected CompilerPass create(AbstractCompiler compiler) {
-      return new UnreachableCodeElimination(compiler, true);
-    }
-  };
+        @Override
+        protected CompilerPass create(AbstractCompiler compiler) {
+          return new UnreachableCodeElimination(compiler, true);
+        }
+
+        @Override
+        protected FeatureSet featureSet() {
+          return ES8;
+        }
+      };
 
   /**
    * Use data flow analysis to remove dead branches.

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

@@ -510,6 +510,10 @@ public void updateSideEffectsForNode(
         if (node.hasChildren() && !NodeUtil.evaluatesToLocalValue(node.getFirstChild())) {
           sideEffectInfo.setTaintsReturn();
         }
+      } else if (node.isYield()) {
+        if (node.hasChildren() && !NodeUtil.evaluatesToLocalValue(node.getFirstChild())) {
+          sideEffectInfo.setTaintsReturn();
+        }
       } else {
         throw new IllegalArgumentException("Unhandled side effect node type " + node.getToken());
       }

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

@@ -202,8 +202,8 @@ private void removeDeadExprStatementSafely(Node n) {
       }
 
       // TODO(user): This is a problem with removeNoOpStatements.
-      // Every expression in a FOR-IN header looks side effect free on its own.
-      if (parent.isForIn()) {
+      // Every expression in a FOR-IN or FOR-OF header looks side effect free on its own.
+      if (NodeUtil.isEnhancedFor(parent)) {
         return;
       }
 

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

@@ -20,7 +20,6 @@
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
-import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
 import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
 import com.google.javascript.rhino.Node;
 import java.util.ArrayList;
@@ -1608,6 +1607,34 @@ public void testMutatesArgumentsArray3() throws Exception {
     assertNoPureCalls(source);
   }
 
+  public void testCallGenerator1() {
+    this.mode = TypeInferenceMode.NEITHER; // type check for yield not yet implemented
+    String source =
+        LINE_JOINER.join(
+            "var x = 0;",
+            "function* f() {",
+            "  x = 2",
+            "  while (true) {",
+            "    yield x;",
+            "  }",
+            "}",
+            "var g = f();");
+    assertNoPureCalls(source);
+    Node lastRoot = getLastCompiler().getRoot().getLastChild();
+    Node call = findQualifiedNameNode("f", lastRoot).getParent();
+    assertThat(call.isNoSideEffectsCall()).isFalse();
+    assertEquals(
+        new Node.SideEffectFlags().setReturnsTainted().valueOf(), call.getSideEffectFlags());
+  }
+
+  public void testCallGenerator2() {
+    this.mode = TypeInferenceMode.NEITHER; // type check for yield not yet implemented
+    String source =
+        LINE_JOINER.join(
+            "function* f() {", "  while (true) {", "    yield 1;", "  }", "}", "var g = f()");
+    assertPureCallsMarked(source, ImmutableList.of("f"));
+  }
+
   public void testCallFunctionFOrG() throws Exception {
     String source = LINE_JOINER.join(
         "function f(){}",
@@ -1839,13 +1866,6 @@ void assertPureCallsMarked(String source, List<String> expected) {
   }
 
   void assertPureCallsMarked(String source, List<String> expected, DiagnosticType warning) {
-    assertPureCallsMarked(source, expected, warning, LanguageMode.ECMASCRIPT_2015);
-    assertPureCallsMarked(source, expected, warning, LanguageMode.ECMASCRIPT5);
-  }
-
-  void assertPureCallsMarked(
-      String source, List<String> expected, DiagnosticType warning, LanguageMode mode) {
-    setAcceptedLanguage(mode);
     if (warning != null) {
       testSame(source, warning);
     } else {
@@ -1854,13 +1874,8 @@ void assertPureCallsMarked(
     assertEquals(expected, noSideEffectCalls);
   }
 
-  void checkLocalityOfMarkedCalls(String source, List<String> expected) {
-    checkLocalityOfMarkedCalls(source, expected, LanguageMode.ECMASCRIPT_2015);
-    checkLocalityOfMarkedCalls(source, expected, LanguageMode.ECMASCRIPT5);
-  }
 
-  void checkLocalityOfMarkedCalls(String source, List<String> expected, LanguageMode mode) {
-    setAcceptedLanguage(mode);
+  void checkLocalityOfMarkedCalls(String source, List<String> expected) {
     testSame(source);
     assertEquals(expected, localResultCalls);
   }

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

@@ -428,4 +428,55 @@ public void testIssue1001() throws Exception {
     test("function f(x) { x.property = 3; } new f({})",
          "function f(x) { x.property = 3; }");
   }
+
+  public void testLetConstBlocks() {
+    test("function f() {return 1; let a; }", "function f() {return 1;}");
+
+    test("function f() {return 1; const a = 1; }", "function f() {return 1;}");
+
+    test(
+        "function f() { x = 1; {let g; return x} let y}",
+        "function f() { x = 1; {let g; return x;}} ");
+  }
+
+  public void testArrowFunctions() {
+    test("f(x => {return x; j = 1})", "f(x => {return x;})");
+
+    testSame("f( () => {return 1;})");
+  }
+
+  public void testGenerators() {
+    test(
+        LINE_JOINER.join(
+            "function* f() {", "  while(true) {", "    yield 1;", "  }", "  x = 1;", "}"),
+        LINE_JOINER.join("function* f() {", "  while(true) {", "    yield 1;", "  }", "}"));
+
+    testSame(LINE_JOINER.join("function* f() {", "  while(true) {", "    yield 1;", "  }", "}"));
+
+    testSame(
+        LINE_JOINER.join(
+            "function* f() {",
+            "  let i = 0;",
+            "  while (true) {",
+            "    if (i < 10) {",
+            "      yield i;",
+            "    } else {",
+            "      break;",
+            "    }",
+            "  }",
+            "  let x = 1;",
+            "}"));
+  }
+
+  public void testForOf() {
+    test("for(x of i){ 1; }", "for(x of i) {}");
+
+    testSame("for(x of i){}");
+  }
+
+  public void testRemoveUselessTemplateStrings() {
+    test("`hi`", "");
+
+    testSame("`hello visitor # ${i++}`");
+  }
 }