Source selection API abstraction first cut.

pkg/analyzer_experimental/test/services/formatter_test.dart

Index: pkg/analyzer_experimental/test/services/formatter_test.dart
===================================================================
--- pkg/analyzer_experimental/test/services/formatter_test.dart	(revision 27530)
+++ pkg/analyzer_experimental/test/services/formatter_test.dart	(working copy)
@@ -767,7 +767,8 @@
     test('initialIndent', () {
       var formatter = new CodeFormatter(
           new FormatterOptions(initialIndentationLevel: 2));
-      var formattedSource = formatter.format(CodeKind.STATEMENT, 'var x;');
+      var formattedSource = 
+          formatter.format(CodeKind.STATEMENT, 'var x;').source;
       expect(formattedSource, startsWith('    '));
     });
 
@@ -955,10 +956,10 @@
 }
 
 String formatCU(src, {options: const FormatterOptions()}) =>
-    new CodeFormatter(options).format(CodeKind.COMPILATION_UNIT, src);
+    new CodeFormatter(options).format(CodeKind.COMPILATION_UNIT, src).source;
 
 String formatStatement(src, {options: const FormatterOptions()}) =>
-    new CodeFormatter(options).format(CodeKind.STATEMENT, src);
+    new CodeFormatter(options).format(CodeKind.STATEMENT, src).source;
 
 Token tokenize(String str) => new StringScanner(null, str, null).tokenize();