Update the test runner to use the new dart:io API

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
diff --git a/tools/testing/dart/multitest.dart b/tools/testing/dart/multitest.dart
index 585502c298e26c49677de6412a03d113da8f2fd9..44ba7731535caa2ed209bd8a52bad8d174659faa 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -137,12 +137,11 @@ class _Annotation {
       return null;
     }
     var annotation = new _Annotation();
-    var parts = line.split('///')[1].split(':')
-        .mappedBy((s) => s.trim()).toList();
+    var parts = line.split('///')[1].split(':').map((s) => s.trim()).toList();
     annotation.key = parts[0];
     annotation.rest = parts[1];
     annotation.outcomesList = annotation.rest.split(',')
-        .mappedBy((s) => s.trim()).toList();
+        .map((s) => s.trim()).toList();
     return annotation;
   }
 }
@@ -151,7 +150,7 @@ class _Annotation {
 // the generated tests.
 Set<Path> _findAllRelativeImports(Path topLibrary) {
   Set<Path> toSearch = new Set<Path>.from([topLibrary]);
-  Set<Path> foundImports = new HashSet<Path>();
+  Set<Path> foundImports = new Set<Path>();
   Path libraryDir = topLibrary.directoryPath;
   // Matches #import( or #source( followed by " or ' followed by anything
   // except dart:, dart-ext: or /, at the beginning of a line.
@@ -159,7 +158,7 @@ Set<Path> _findAllRelativeImports(Path topLibrary) {
       '^#(import|source)[(]["\'](?!(dart:|dart-ext:|/))([^"\']*)["\']');
   while (!toSearch.isEmpty) {
     var thisPass = toSearch;
-    toSearch = new HashSet<Path>();
+    toSearch = new Set<Path>();
     for (Path filename in thisPass) {
       File f = new File.fromPath(filename);
       for (String line in f.readAsLinesSync()) {
@@ -219,8 +218,7 @@ Future doMultitest(Path filePath, String outputDir, Path suiteDir,
 
       file.createSync();
       RandomAccessFile openedFile = file.openSync(FileMode.WRITE);
-      var bytes = tests[key].charCodes;
-      openedFile.writeListSync(bytes, 0, bytes.length);
+      openedFile.writeStringSync(tests[key]);
       openedFile.closeSync();
       Set<String> outcome = outcomes[key];
       bool enableFatalTypeErrors = outcome.contains('static type warning');