Don't throw IllegalStateException for unusual class declarations.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138880645

src/com/google/javascript/jscomp/Es6ConvertSuperConstructorCalls.java

@@ -228,7 +228,8 @@ private boolean isKnownToReturnOnlyUndefined(String functionQName) {
     } else if (declaredValue.isQualifiedName()) {
       return isKnownToReturnOnlyUndefined(declaredValue.getQualifiedName());
     } else {
-      throw new IllegalStateException("Unexpected value: " + declaredValue.toStringTree());
+      // TODO(bradfordcsmith): What cases are these? Can we do better?
+      return false;
     }
   }