Add an "onPlatform" parameter to test() and group().

lib/src/backend/metadata.dart

Index: lib/src/backend/metadata.dart
diff --git a/lib/src/backend/metadata.dart b/lib/src/backend/metadata.dart
index ecce4404601c0f59672c6a64e411d9409c09b7f8..0bcf5f880b66c6b7fb2facc0c30cd375cecdb110 100644
--- a/lib/src/backend/metadata.dart
+++ b/lib/src/backend/metadata.dart
@@ -4,7 +4,13 @@
 
 library test.backend.metadata;
 
+import 'dart:collection';
+
+import '../backend/operating_system.dart';
+import '../backend/test_platform.dart';
+import '../frontend/skip.dart';
 import '../frontend/timeout.dart';
+import '../utils.dart';
 import 'platform_selector.dart';
 
 /// Metadata for a test or test suite.
@@ -24,26 +30,81 @@ class Metadata {
   /// The reason the test or suite should be skipped, if given.
   final String skipReason;
 
+  /// Platform-specific metadata.
+  ///
+  /// Each key identifies a platform, and its value identifies the specific
+  /// metadata for that platform. These can be applied by calling [forPlatform].
+  final Map<PlatformSelector, Metadata> onPlatform;
+
+  /// Parses a user-provided map into the value for [onPlatform].
+  static Map<PlatformSelector, Metadata> _parseOnPlatform(
+      Map<String, dynamic> onPlatform) {
+    if (onPlatform == null) return {};
+
+    var result = {};
+    onPlatform.forEach((platform, metadata) {
+      if (metadata is Timeout || metadata is Skip) {
+        metadata = [metadata];
+      } else if (metadata is! List) {
+        throw new ArgumentError('Metadata for platform "$platform" must be a '
+            'Timeout, Skip, or List of those; was "$metadata".');
+      }
+
+      var selector = new PlatformSelector.parse(platform);
+
+      var timeout;
+      var skip;
+      for (var metadatum in metadata) {
+        if (metadatum is Timeout) {
+          if (timeout != null) {
+            throw new ArgumentError('Only a single Timeout may be declared for '
+                '"$platform".');
+          }
+
+          timeout = metadatum;
+        } else if (metadatum is Skip) {
+          if (skip != null) {
+            throw new ArgumentError('Only a single Skip may be declared for '
+                '"$platform".');
+          }
+
+          skip = metadatum.reason == null ? true : metadatum.reason;
+        } else {
+          throw new ArgumentError('Metadata for platform "$platform" must be a '
+              'Timeout, Skip, or List of those; was "$metadata".');
+        }
+      }
+
+      result[selector] = new Metadata.parse(timeout: timeout, skip: skip);
+    });
+    return result;
+  }
+
   /// Creates new Metadata.
   ///
   /// [testOn] defaults to [PlatformSelector.all].
   Metadata({PlatformSelector testOn, Timeout timeout, bool skip: false,
-          this.skipReason})
+          this.skipReason, Map<PlatformSelector, Metadata> onPlatform})
       : testOn = testOn == null ? PlatformSelector.all : testOn,
         timeout = timeout == null ? const Timeout.factor(1) : timeout,
-        skip = skip;
+        skip = skip,
+        onPlatform = onPlatform == null
+            ? const {}
+            : new UnmodifiableMapView(onPlatform);
 
-  /// Creates a new Metadata, but with fields parsed from strings where
-  /// applicable.
+  /// Creates a new Metadata, but with fields parsed from caller-friendly values
+  /// where applicable.
   ///
   /// Throws a [FormatException] if any field is invalid.
-  Metadata.parse({String testOn, Timeout timeout, skip})
+  Metadata.parse({String testOn, Timeout timeout, skip,
+          Map<String, dynamic> onPlatform})
       : testOn = testOn == null
             ? PlatformSelector.all
             : new PlatformSelector.parse(testOn),
         timeout = timeout == null ? const Timeout.factor(1) : timeout,
         skip = skip != null && skip != false,
-        skipReason = skip is String ? skip : null {
+        skipReason = skip is String ? skip : null,
+        onPlatform = _parseOnPlatform(onPlatform) {
     if (skip != null && skip is! String && skip is! bool) {
       throw new ArgumentError(
           '"skip" must be a String or a bool, was "$skip".');
@@ -52,15 +113,18 @@ class Metadata {
 
   /// Dezerializes the result of [Metadata.serialize] into a new [Metadata].
   Metadata.deserialize(serialized)
-      : this.parse(
-          testOn: serialized['testOn'],
-          timeout: serialized['timeout']['duration'] == null
-              ? new Timeout.factor(serialized['timeout']['scaleFactor'])
-              : new Timeout(new Duration(
-                  microseconds: serialized['timeout']['duration'])),
-          skip: serialized['skipReason'] == null
-              ? serialized['skip']
-              : serialized['skipReason']);
+      : testOn = serialized['testOn'] == null
+            ? PlatformSelector.all
+            : new PlatformSelector.parse(serialized['testOn']),
+        timeout = serialized['timeout']['duration'] == null
+            ? new Timeout.factor(serialized['timeout']['scaleFactor'])
+            : new Timeout(new Duration(
+                microseconds: serialized['timeout']['duration'])),
+        skip = serialized['skip'],
+        skipReason = serialized['skipReason'],
+        onPlatform = new Map.fromIterable(serialized['onPlatform'],
+            key: (pair) => new PlatformSelector.parse(pair.first),
+            value: (pair) => new Metadata.deserialize(pair.last));
 
   /// Return a new [Metadata] that merges [this] with [other].
   ///
@@ -70,19 +134,52 @@ class Metadata {
           testOn: testOn.intersect(other.testOn),
           timeout: timeout.merge(other.timeout),
           skip: skip || other.skip,
-          skipReason: other.skipReason == null ? skipReason : other.skipReason);
+          skipReason: other.skipReason == null ? skipReason : other.skipReason,
+          onPlatform: mergeMaps(onPlatform, other.onPlatform));
+
+  /// Returns a copy of [this] with the given fields changed.
+  Metadata change({PlatformSelector testOn, Timeout timeout, bool skip,
+      String skipReason, Map<PlatformSelector, Metadata> onPlatform}) {
+    if (testOn == null) testOn = this.testOn;
+    if (timeout == null) timeout = this.timeout;
+    if (skip == null) skip = this.skip;
+    if (skipReason == null) skipReason = this.skipReason;
+    if (onPlatform == null) onPlatform = this.onPlatform;
+    return new Metadata(testOn: testOn, timeout: timeout, skip: skip,
+        skipReason: skipReason, onPlatform: onPlatform);
+  }
+
+  /// Returns a copy of [this] with all platform-specific metadata from
+  /// [onPlatform] resolved.
+  Metadata forPlatform(TestPlatform platform, {OperatingSystem os}) {
+    var metadata = this;
+    onPlatform.forEach((platformSelector, platformMetadata) {
+      if (!platformSelector.evaluate(platform, os: os)) return;
+      metadata = metadata.merge(platformMetadata);
+    });
+    return metadata.change(onPlatform: {});
+  }
 
   /// Serializes [this] into a JSON-safe object that can be deserialized using
   /// [new Metadata.deserialize].
-  serialize() => {
-    'testOn': testOn == PlatformSelector.all ? null : testOn.toString(),
-    'timeout': {
-      'duration': timeout.duration == null
-          ? null
-          : timeout.duration.inMicroseconds,
-      'scaleFactor': timeout.scaleFactor
-    },
-    'skip': skip,
-    'skipReason': skipReason
-  };
+  serialize() {
+    // Make this a list to guarantee that the order is preserved.
+    var serializedOnPlatform = [];
+    onPlatform.forEach((key, value) {
+      serializedOnPlatform.add([key.toString(), value.serialize()]);
+    });
+
+    return {
+      'testOn': testOn == PlatformSelector.all ? null : testOn.toString(),
+      'timeout': {
+        'duration': timeout.duration == null
+            ? null
+            : timeout.duration.inMicroseconds,
+        'scaleFactor': timeout.scaleFactor
+      },
+      'skip': skip,
+      'skipReason': skipReason,
+      'onPlatform': serializedOnPlatform
+    };
+  }
 }