Don't crash on classes with "empty" children (i.e. extraneous semicol…

…ons in between methods in the class body)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161271286

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

@@ -177,6 +177,9 @@ private static void checkObjectLiteralOrClass(NodeTraversal t, Node n) {
     for (Node key = n.getFirstChild();
          key != null;
          key = key.getNext()) {
+      if (key.isEmpty()) {
+        continue;
+      }
       String keyName = key.getString();
       if (!key.isSetterDef()) {
         // normal property and getter cases

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

@@ -201,12 +201,13 @@ public void testDuplicateObjectLiteralKey() {
          StrictModeCheck.DUPLICATE_OBJECT_KEY);
 
     testSame(
-        "'use strict';\n" +
-        "/** @constructor */ function App() {}\n" +
-        "App.prototype = {\n" +
-        "  get appData() { return this.appData_; },\n" +
-        "  set appData(data) { this.appData_ = data; }\n" +
-        "};");
+        LINE_JOINER.join(
+            "'use strict';",
+            "/** @constructor */ function App() {}",
+            "App.prototype = {",
+            "  get appData() { return this.appData_; },",
+            "  set appData(data) { this.appData_ = data; }",
+            "};"));
 
     this.mode = TypeInferenceMode.NEITHER;
     testError("var x = {a: 2, a(){}}", StrictModeCheck.DUPLICATE_OBJECT_KEY);
@@ -327,6 +328,11 @@ public void testClass() {
         "}"), StrictModeCheck.ARGUMENTS_CALLER_FORBIDDEN);
   }
 
+  public void testClassWithEmptyMembers() {
+    this.mode = TypeInferenceMode.NEITHER;
+    testError("class Foo { dup() {}; dup() {}; }", StrictModeCheck.DUPLICATE_CLASS_METHODS);
+  }
+
   /**
    * If the LanguageMode is ES2015 or higher, strict mode violations are automatically upgraded to
    * errors, so set it to ES5 to get a warning.