Add Chain.foldFrames to pkg/stack_trace.

pkg/stack_trace/test/chain_test.dart

Index: pkg/stack_trace/test/chain_test.dart
diff --git a/pkg/stack_trace/test/chain_test.dart b/pkg/stack_trace/test/chain_test.dart
index 9c166909e1f65f741a0d774bb445d28ffe7ed355..d9915ebf1f9767c21b1c8890bd95471d73730b3f 100644
--- a/pkg/stack_trace/test/chain_test.dart
+++ b/pkg/stack_trace/test/chain_test.dart
@@ -387,7 +387,7 @@ void main() {
           'dart:core             Bar.baz\n'));
     });
 
-    test("doesn't return in an empty chain", () {
+    test("doesn't return an empty chain", () {
       var chain = new Chain([
         new Trace.parse(
             'dart:core 10:11                             Foo.bar\n'
@@ -403,6 +403,67 @@ void main() {
     });
   });
 
+  group('Chain.foldFrames', () {
+    test('folds each trace', () {
+      var chain = new Chain([
+        new Trace.parse(
+            'a.dart 10:11  Foo.bar\n'
+            'a.dart 10:11  Bar.baz\n'
+            'b.dart 10:11  Bang.qux\n'
+            'a.dart 10:11  Zip.zap\n'
+            'a.dart 10:11  Zop.zoop'),
+        new Trace.parse(
+            'a.dart 10:11  Foo.bar\n'
+            'a.dart 10:11  Bar.baz\n'
+            'a.dart 10:11  Bang.qux\n'
+            'a.dart 10:11  Zip.zap\n'
+            'b.dart 10:11  Zop.zoop')
+      ]);
+
+      var folded = chain.foldFrames((frame) => frame.library == 'a.dart');
+      expect(folded.toString(), equals(
+          'a.dart 10:11  Bar.baz\n'
+          'b.dart 10:11  Bang.qux\n'
+          'a.dart 10:11  Zop.zoop\n'
+          '===== asynchronous gap ===========================\n'
+          'a.dart 10:11  Zip.zap\n'
+          'b.dart 10:11  Zop.zoop\n'));
+    });
+
+    test('eliminates completely-folded traces', () {
+      var chain = new Chain([
+        new Trace.parse(
+            'a.dart 10:11  Foo.bar\n'
+            'b.dart 10:11  Bang.qux'),
+        new Trace.parse(
+            'a.dart 10:11  Foo.bar\n'
+            'a.dart 10:11  Bang.qux'),
+        new Trace.parse(
+            'a.dart 10:11  Zip.zap\n'
+            'b.dart 10:11  Zop.zoop')
+      ]);
+
+      var folded = chain.foldFrames((frame) => frame.library == 'a.dart');
+      expect(folded.toString(), equals(
+          'a.dart 10:11  Foo.bar\n'
+          'b.dart 10:11  Bang.qux\n'
+          '===== asynchronous gap ===========================\n'
+          'a.dart 10:11  Zip.zap\n'
+          'b.dart 10:11  Zop.zoop\n'));
+    });
+
+    test("doesn't return an empty trace", () {
+      var chain = new Chain([
+        new Trace.parse(
+            'a.dart 10:11  Foo.bar\n'
+            'a.dart 10:11  Bang.qux')
+      ]);
+
+      var folded = chain.foldFrames((frame) => frame.library == 'a.dart');
+      expect(folded.toString(), equals('a.dart 10:11  Bang.qux\n'));
+    });
+  });
+
   test('Chain.toTrace eliminates asynchronous gaps', () {
     var trace = new Chain([
       new Trace.parse(