Move constant systems into their own libraries.

tests/compiler/dart2js/value_range2_test.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.
 
 import "package:expect/expect.dart";
 import "package:compiler/src/ssa/ssa.dart";
-import "package:compiler/src/dart2jslib.dart";
-import "package:compiler/src/js_backend/js_backend.dart";
+import "package:compiler/src/js_backend/constant_system_javascript.dart";
 
 ValueRangeInfo info = new ValueRangeInfo(const JavaScriptConstantSystem());
 
 Value instructionValue = info.newInstructionValue(new HBreak(null));
 Value lengthValue = info.newPositiveValue(new HBreak(null));
 
 Range createSingleRange(Value value) => info.newNormalizedRange(value, value);
 
 Range createSingleIntRange(int value) {
   return createSingleRange(info.newIntValue(value));
@@ skipping 357 matching lines @@
   checkSubRange(_0_instruction, _0_length, null, _0_instruction.upper);
   checkSubRange(_0_instruction, _0_instruction, null, _0_instruction.upper);
 }
 
 main() {
   HInstruction.idCounter = 0;
   testAnd();
   testSub();
   testNegate();
 }