When calling super['someProperty']() (as opposed to super.someProperty()) report an error instead of crashing.

Fixes #1184
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=104729872

Author: tbreisacher

src/com/google/javascript/jscomp/Es6ConvertSuper.java

@@ -108,7 +108,9 @@ private void visitSuper(Node node, Node parent) {
       enclosingCall = parent.getParent();
       potentialCallee = parent;
     }
-    if (!enclosingCall.isCall() || enclosingCall.getFirstChild() != potentialCallee) {
+    if (!enclosingCall.isCall()
+        || enclosingCall.getFirstChild() != potentialCallee
+        || enclosingCall.getFirstChild().isGetElem()) {
       compiler.report(JSError.make(node, CANNOT_CONVERT_YET,
           "Only calls to super or to a method of super are supported."));
       return;

test/com/google/javascript/jscomp/Es6ToEs3ConverterTest.java

@@ -515,6 +515,21 @@ public void testSuperCall() {
             "}"));
   }
 
+  public void testComputedSuper() {
+    testError(
+        LINE_JOINER.join(
+            "class Foo {",
+            "  ['m']() { return 1; }",
+            "}",
+            "",
+            "class Bar extends Foo {",
+            "  ['m']() {",
+            "    return super['m']() + 1;",
+            "  }",
+            "}"),
+        CANNOT_CONVERT_YET);
+  }
+
   public void testMultiNameClass() {
     test("var F = class G {}",
         "/** @constructor @struct */ var F = function() {};");