[NTI] Fix crash that comes up in mixin-style code.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145826102

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

@@ -624,7 +624,11 @@ private TypeEnv getTypeEnvFromDeclaredTypes() {
     }
     if (currentScope.isFunction()) {
       Node fn = currentScope.getRoot();
-      if (!currentScope.hasThis() && NodeUtil.referencesSuper(fn)) {
+      if (!currentScope.hasThis()
+          // Can't use NodeUtil.referencesSuper here because that function is correct only
+          // on valid ASTs, but here we may have an invalid AST that contains super inside
+          // a function.
+          && NodeUtil.containsType(fn.getLastChild(), Token.SUPER, NodeUtil.MATCH_NOT_FUNCTION)) {
         // This function is a static method on some class. To do lookups of the
         // class name, we add the root of the qualified name to the environment.
         Node funNameNode = NodeUtil.getBestLValue(fn);

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

@@ -336,4 +336,14 @@ public void testAbstractMethodCalls() {
         "  foo() { super.foo(); }",
         "}"));
   }
+
+  // super is handled natively in both type checkers, which results in ASTs that
+  // are temporarily invalid: a super call that is not in a class.
+  // Avoid crashing.
+  public void testDontCrashWithInvalidIntermediateASTwithSuper() {
+    typeCheck(LINE_JOINER.join(
+        "class Foo {}",
+        "function g(x) {}",
+        "g(function f(x) { return class extends Foo {} });"));
+  }
 }