Fix warnings in analyzer_experimental

pkg/analyzer_experimental/lib/analyzer.dart

 // Copyright (c) 2013, 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 analyzer;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as pathos;
 
 import 'src/error.dart';
 import 'src/generated/ast.dart';
 import 'src/generated/error.dart';
-import 'src/generated/java_io.dart';
 import 'src/generated/parser.dart';
 import 'src/generated/scanner.dart';
 import 'src/generated/source_io.dart';
 
 export 'src/error.dart';
 export 'src/generated/ast.dart';
 export 'src/generated/error.dart';
 export 'src/generated/utilities_dart.dart';
 
 /// Parses a Dart file into an AST.
@@ skipping 17 matching lines @@
   var unit = parser.parseCompilationUnit(token);
   unit.lineInfo = new LineInfo(scanner.lineStarts);
 
   if (errorCollector.hasErrors) throw errorCollector.group;
 
   return unit;
 }
 
 /// Converts an AST node representing a string literal into a [String].
 String stringLiteralToString(StringLiteral literal) {
-  if (literal is AdjacentStrings) {
-    return literal.strings.map(stringLiteralToString).join();
-  } else if (literal is SimpleStringLiteral) {
-    return literal.value;
-  } else {
-    throw new ArgumentError("Can't convert $literal to a Dart string.");
-  }
+  return literal.stringValue;
 }
 
 /// A simple error listener that collects errors into an [AnalysisErrorGroup].
 class _ErrorCollector extends AnalysisErrorListener {
   final _errors = <AnalysisError>[];
 
   /// Whether any errors where collected.
   bool get hasErrors => !_errors.isEmpty;
 
   /// The group of errors collected.
   AnalyzerErrorGroup get group =>
     new AnalyzerErrorGroup.fromAnalysisErrors(_errors);
 
   _ErrorCollector();
 
   void onError(AnalysisError error) => _errors.add(error);
 }