Remove factory body in *.fromEnvironment, and implement this same behavior

tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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.
 
 // Library for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 const DEFAULT_PLATFORM_CONFIG = """
 [libraries]
@@ skipping 262 matching lines @@
       'throwAbstractClassInstantiationError(className) {}',
   'checkConcurrentModificationError':
       'checkConcurrentModificationError(collection) {}',
   'throwConcurrentModificationError':
       'throwConcurrentModificationError(collection) {}',
   'throwCyclicInit': 'throwCyclicInit(staticName) {}',
   'throwExpression': 'throwExpression(e) {}',
   'throwNoSuchMethod':
       'throwNoSuchMethod(obj, name, arguments, expectedArgumentNames) {}',
   'throwRuntimeError': 'throwRuntimeError(message) {}',
+  'throwUnsupportedError': 'throwUnsupportedError(message) {}',
   'throwTypeError': 'throwTypeError(message) {}',
   'TypeImpl': 'class TypeImpl {}',
   'TypeVariable': '''class TypeVariable {
     final Type owner;
     final String name;
     final int bound;
     const TypeVariable(this.owner, this.name, this.bound);
   }''',
   'unwrapException': 'unwrapException(e) {}',
   'voidTypeCheck': 'voidTypeCheck(value) {}',
@@ skipping 191 matching lines @@
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
   '_version': 'const _version = "0.0.1+1";',
 };