Convert most of the EngineTestCase.createSource invocations into multi-line String literals.

pkg/analyzer/test/task/task_dart_test.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 test.engine.task.dart;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 43 matching lines @@
     source.generateExceptionOnRead = true;
     InternalAnalysisContext context = new AnalysisContextImpl();
     context.sourceFactory = new SourceFactory([new FileUriResolver()]);
     CompilationUnit unit = parseUnit(context, source, "");
     BuildUnitElementTask task =
         new BuildUnitElementTask(context, null, source, unit);
     task.perform(new BuildUnitElementTaskTV_perform_exception());
   }
 
   void test_perform_valid() {
-    var content = EngineTestCase.createSource(["library lib;", "class A {}"]);
+    var content = """
+library lib;
+class A {}""";
     Source source = new TestSource('/test.dart', content);
     InternalAnalysisContext context = AnalysisContextFactory.contextWithCore();
     CompilationUnit unit = parseUnit(context, source, content);
     BuildUnitElementTask task =
         new BuildUnitElementTask(context, source, source, unit);
     task.perform(new BuildUnitElementTaskTV_perform_valid(source, unit));
   }
 
   CompilationUnit parseUnit(InternalAnalysisContext context, Source source, String content) {
     ScanDartTask scanTask = new ScanDartTask(
@@ skipping 37 matching lines @@
     if (exception != null) {
       throw exception;
     }
     expect(task.source, equals(source));
     expect(task.library, equals(source));
     expect(task.unit, equals(unit));
     expect(task.unitElement, isNotNull);
     return true;
   }
 }