| Index: tool/input_sdk/private/js_array.dart
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| diff --git a/tool/input_sdk/private/js_array.dart b/tool/input_sdk/private/js_array.dart
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| index c9aab5f74eea2f6acccabea2f56a094e5e2916cd..fae79523869fa28d40737ebf16ce83598a065c05 100644
 | 
| --- a/tool/input_sdk/private/js_array.dart
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| +++ b/tool/input_sdk/private/js_array.dart
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| @@ -85,7 +85,7 @@ class JSArray<E> implements List<E>, JSIndexable {
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|    }
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|  
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|    checkGrowable(reason) {
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| -    if (this is !JSExtendableArray) {
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| +    if (JS('bool', r'#.fixed$length', this)) {
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|        throw new UnsupportedError(reason);
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|      }
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|    }
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| 
 |