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

pkg/analyzer/test/task/task_dart_test.dart

Index: pkg/analyzer/test/task/task_dart_test.dart
diff --git a/pkg/analyzer/test/task/task_dart_test.dart b/pkg/analyzer/test/task/task_dart_test.dart
index b9e86ecc9290439ebf3bc1a5e45612c80509201f..9a44b7d1e9436fa759d12205efcf732ebbc4d65e 100644
--- a/pkg/analyzer/test/task/task_dart_test.dart
+++ b/pkg/analyzer/test/task/task_dart_test.dart
@@ -61,7 +61,9 @@ class BuildUnitElementTaskTest extends EngineTestCase {
   }
 
   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);