Add resource tests.

tests/corelib/package_resource_test.dart

+// Copyright (c) 2015, 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.
+
+const sampleText = "Sample text file.";
+
+main() async {
+  const uriText = "package:package_test_data/resources/sample.txt";
+  const resource = const Resource(uriText);
+
+  if (resource.uri != Uri.parse(uriText)) {
+    throw "Incorrect URI: ${resource.uri}";
+  }
+
+  var text = await resource.readAsString();
+  if (!text.startsWith("Sample text file.")) {
+    throw "Incorrect reading of text file: $text";
+  }
+
+  var bytes = await resource.readAsBytes();
+  if (!compareBytes(bytes, sampleText.codeUnits)) {
+    throw "Incorrect reading of bytes: $bytes";
+  }
+
+  var streamBytes = [];
+  await for (var byteSlice in resource.openRead()) {
+    streamBytes.addAll(byteSlice);
+  }
+  if (!compareBytes(streamBytes, sampleText.codeUnits)) {
+    throw "Incorrect reading of bytes: $bytes";
+  }
+
+  if (!compareBytes(streamBytes, bytes)) {
+    throw "Inconsistent reading of bytes: $bytes / $streamBytes";
+  }
+}
+
+/// Checks that [bytes] starts with [expectedBytes].
+///
+/// The bytes may be longer (because the test file is a text file and its
+/// terminating line ending may be mangled on some platforms).
+bool compareBytes(bytes, expectedBytes) {
+  if (bytes.length < expectedBytes.length) return false;
+  for (int i = 0; i < expectedBytes.length; i++) {
+    if (bytes[i] != expectedBytes[i]) return false;
+  }
+  return true;
+}