Remove DartString from constants.

pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart

 // Copyright (c) 2015, 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.
 
 /// Analysis to determine how to generate code for `LookupMap`s.
 library compiler.src.js_backend.lookup_map_analysis;
 
 import 'package:pub_semver/pub_semver.dart';
 
 import '../common.dart';
@@ skipping 128 matching lines @@
     if (value == null) {
       reporter.reportHintMessage(lookupMapVersionVariable,
           MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP);
       return const LookupMapAnalysis._();
     }
 
     // TODO(sigmund): add proper version resolution using the pub_semver package
     // when we introduce the next version.
     Version version;
     try {
-      version = new Version.parse(value.primitiveValue.slowToString());
+      version = new Version.parse(value.primitiveValue);
     } catch (e) {}
 
     if (version == null || !_validLookupMapVersionConstraint.allows(version)) {
       reporter.reportHintMessage(lookupMapVersionVariable,
           MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP);
       return const LookupMapAnalysis._();
     }
 
     ClassEntity typeLookupMapClass =
         elementEnvironment.lookupClass(analysis.lookupMapLibrary, 'LookupMap');
@@ skipping 337 matching lines @@
         original.fields[analysis._valueField] = keyValuePairs[1];
       }
     } else {
       original.fields[analysis._entriesField] =
           new ListConstantValue(listType, keyValuePairs);
     }
   }
 }
 
 final _validLookupMapVersionConstraint = new VersionConstraint.parse('^0.0.1');