green bots: fix analyzer warning in lookupmap

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 '../common/registry.dart' show Registry;
 import '../compiler.dart' show Compiler;
 import '../diagnostics/messages.dart' show MessageKind;
 import '../constants/values.dart' show
      ConstantValue,
      ConstructedConstantValue,
      ListConstantValue,
      NullConstantValue,
+     StringConstantValue,
      TypeConstantValue;
 import '../dart_types.dart' show DartType;
 import '../elements/elements.dart' show
     ClassElement,
     Element,
     Elements,
     FieldElement,
     FunctionElement,
     FunctionSignature,
     LibraryElement,
@@ skipping 118 matching lines @@
   }
 
   /// Checks if the version of lookup_map is valid, and if so, enable this
   /// analysis during codegen.
   void onCodegenStart() {
     _inCodegen = true;
     if (lookupMapVersionVariable == null) return;
 
     // At this point, the lookupMapVersionVariable should be resolved and it's
     // constant value should be available.
-    ConstantValue value =
+    StringConstantValue value =
         backend.constants.getConstantValueForVariable(lookupMapVersionVariable);
     if (value == null) {
       backend.compiler.reportInfo(lookupMapVersionVariable,
           MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP);
       return;
     }
 
     // TODO(sigmund): add proper version resolution using the pub_semver package
     // when we introduce the next version.
     String version = value.primitiveValue.slowToString();
@@ skipping 257 matching lines @@
       if (keyValuePairs.length == 2) {
         original.fields[analysis.keyField] = keyValuePairs[0];
         original.fields[analysis.valueField] = keyValuePairs[1];
       }
     } else {
       original.fields[analysis.entriesField] =
           new ListConstantValue(listType, keyValuePairs);
     }
   }
 }