pub run test fails if dart is not in your path

These also might fail if the `dart` in your path was
of a different version than you expected.  This happens
with Flutter since flutter downloads its own copy of the
Dart SDK which may be differnt from the one installed
globally on your disk.

In my case, I just was using an explicit path to `pub`
for `pub run test` and was surprised to see test failures.

@keertip

Author: Eric Seidel

test/compare_output_test.dart

@@ -60,8 +60,8 @@ void main() {
         tempDir.path
       ];
 
-      var result =
-          Process.runSync('dart', args, workingDirectory: _testPackagePath);
+      var result = Process.runSync(Platform.resolvedExecutable, args,
+          workingDirectory: _testPackagePath);
 
       if (result.exitCode != 0) {
         print(result.exitCode);
@@ -129,8 +129,8 @@ void main() {
         tempDir.path
       ];
 
-      var result =
-          Process.runSync('dart', args, workingDirectory: _testPackagePath);
+      var result = Process.runSync(Platform.resolvedExecutable, args,
+          workingDirectory: _testPackagePath);
 
       if (result.exitCode != 0) {
         print(result.exitCode);

tool/grind.dart

@@ -144,7 +144,7 @@ Future buildSdkDocs() async {
   delete(docsDir);
   log('building SDK docs');
   int sdkDocsGenTime = await _runAsyncTimed(() async {
-    var process = await Process.start('dart', [
+    var process = await Process.start(Platform.resolvedExecutable, [
       'bin/dartdoc.dart',
       '--output',
       '${docsDir.path}',