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

pkg/analyzer/test/generated/element_test.dart

Index: pkg/analyzer/test/generated/element_test.dart
diff --git a/pkg/analyzer/test/generated/element_test.dart b/pkg/analyzer/test/generated/element_test.dart
index a6c1bf1f57daf75507d47bd04ae317b26be2a14f..f9b9c83c95f9900681838f537496b94f9e0be5ba 100644
--- a/pkg/analyzer/test/generated/element_test.dart
+++ b/pkg/analyzer/test/generated/element_test.dart
@@ -128,7 +128,9 @@ class ClassElementImplTest extends EngineTestCase {
   void test_getNode() {
     AnalysisContextHelper contextHelper = new AnalysisContextHelper();
     AnalysisContext context = contextHelper.context;
-    Source source = contextHelper.addSource("/test.dart", EngineTestCase.createSource(["class A {}", "class B {}"]));
+    Source source = contextHelper.addSource("/test.dart", r'''
+class A {}
+class B {}''');
     // prepare CompilationUnitElement
     LibraryElement libraryElement = context.computeLibraryElement(source);
     CompilationUnitElement unitElement = libraryElement.definingCompilationUnit;