Made removeAt errors consistent between dart2js and VM.

runtime/lib/growable_array.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class GrowableObjectArray<T> implements List<T> {
   factory GrowableObjectArray._uninstantiable() {
     throw const UnsupportedOperationException(
         "GrowableObjectArray can only be allocated by the VM");
   }
 
-  E removeAt(int index) {
-    E result = this[index];
+  T removeAt(int index) {
+    if (index is! int) throw new IllegalArgumentException(index);
+    T result = this[index];
+    int newLength = this.length - 1;
     Arrays.copy(this,
                 index + 1,
                 this,
                 index,
-                this.length - index - 1);
-    this.length = this.length - 1;
+                newLength - index);
+    this.length = newLength;
     return result;
   }
   
   void setRange(int start, int length, List<T> from, [int startFrom = 0]) {
     if (length < 0) {
       throw new IllegalArgumentException("negative length $length");
     }
     Arrays.copy(from, startFrom, this, start, length);
   }
 
@@ skipping 209 matching lines @@
   T next() {
     if (!hasNext()) {
       throw const NoMoreElementsException();
     }
     return _array[_pos++];
   }
 
   final GrowableObjectArray<T> _array;
   int _pos;
 }