Print child process' stderr when it fails in full_coverage_test.

tests/standalone/full_coverage_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // These tests fork a second VM process that runs the script
 // ``tools/full-coverage.dart'' and verifies that the tool
 // produces the expeced output.
 
 import 'dart:async';
 import 'dart:convert';
@@ skipping 75 matching lines @@
 destroyEnv(base) => new Directory(base).deleteSync(recursive: true);
 
 
 generateCoverage(String workingDirectory) {
   for (var coverageProg in coverageTests) {
     var progPath = path.join(workingDirectory, coverageProg['name']);
     var script = path.join(progPath, "${coverageProg['name']}.dart");
     var dartArgs = new List.from(dartBaseArgs)
       ..addAll(['--coverage-dir=${progPath}', '${script}']);
     var result = Process.runSync(Platform.executable, dartArgs);
-    expect(result.exitCode, 0);
+    if (result.exitCode != 0) {
+      print("Coverage generator returned exitCode: ${result.exitCode}.");
+      print("stderr:\n${result.stderr}\n");
+      expect(result.exitCode, 0);
+    }
   }
 }
 
 
 Future<Process> convertCoverage(String programDir, String format) {
   var dartArgs = new List.from(dartBaseArgs)
       ..addAll([
         coverageScript,
         '--package-root=${packageRoot}',
         '--in=${programDir}',
@@ skipping 116 matching lines @@
       String programPath = path.join(programDir, "${cTest['name']}.dart");
       testCoverage(programDir, programPath,
                    new LcovDescriptor(programPath),
                    new List.from(cTest['expectedHits']));
       testCoverage(programDir, programPath,
                    new PrettyPrintDescriptor(programPath),
                    new List.from(cTest['expectedHits']));
     });
   });
 }