Fix check for ES5 classes extending ES6 classes

Previously this only worked on transpiled classes because it looked for a Node property (IS_ES6_CLASS) that was added by an earlier transpilation pass.  Add a Node method to also check for actual untranspiled CLASS nodes.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=204974193

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

@@ -2048,8 +2048,8 @@ private void checkConstructor(NodeTraversal t, Node n, FunctionType functionType
       if (n.isFunction()
           && baseConstructor != null
           && baseConstructor.getSource() != null
-          && baseConstructor.getSource().getBooleanProp(Node.IS_ES6_CLASS)
-          && !functionType.getSource().getBooleanProp(Node.IS_ES6_CLASS)) {
+          && baseConstructor.getSource().isEs6Class()
+          && !functionType.getSource().isEs6Class()) {
         // Warn if an ES5 class extends an ES6 class.
         compiler.report(
             t.makeError(

src/com/google/javascript/rhino/Node.java

@@ -2655,6 +2655,11 @@ public final boolean isYieldAll() {
     return getBooleanProp(YIELD_ALL);
   }
 
+  /** Returns true if this is or ever was a CLASS node (i.e. even after transpilation). */
+  public final boolean isEs6Class() {
+    return isClass() || getBooleanProp(IS_ES6_CLASS);
+  }
+
   // There are four values of interest:
   //   global state changes
   //   this state changes

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

@@ -3339,6 +3339,24 @@ public void testClassNewTargetInConstructorNestedArrow() {
             "required: null"));
   }
 
+  public void testClassEs5ClassCannotExtendEs6Class() {
+    testTypes(
+        lines(
+            "class Base {}",
+            "/** @constructor @extends {Base} */",
+            "function Sub() {}"),
+        "ES5 class Sub cannot extend ES6 class Base");
+  }
+
+  public void testClassEs5ClassCanImplementEs6Interface() {
+    testTypes(
+        lines(
+            "/** @interface */",
+            "class Inter {}",
+            "/** @constructor @implements {Inter} */",
+            "function Sub() {}"));
+  }
+
   public void testAsyncFunctionWithoutJSDoc() {
     testTypes("async function f() { return 3; }");
   }