Provide more data on failure

pkg/analyzer/test/generated/java_io_test.dart

Index: pkg/analyzer/test/generated/java_io_test.dart
diff --git a/pkg/analyzer/test/generated/java_io_test.dart b/pkg/analyzer/test/generated/java_io_test.dart
index f920ce386c7ade8f18be0b75c1a8489007841a1a..63e8894b199a078add4cacbac7e24345ad057c79 100644
--- a/pkg/analyzer/test/generated/java_io_test.dart
+++ b/pkg/analyzer/test/generated/java_io_test.dart
@@ -25,7 +25,7 @@ main() {
               .normalize(JavaFile.pathContext.absolute(tempDir.path));
           String path = JavaFile.pathContext.join(tempPath, 'foo.dart');
           // we use an absolute path
-          expect(JavaFile.pathContext.isAbsolute(path), isTrue);
+          expect(JavaFile.pathContext.isAbsolute(path), isTrue, reason: '"$path" is not absolute');
           // test that toURI() returns an absolute URI
           Uri uri = new JavaFile(path).toURI();
           expect(uri.isAbsolute, isTrue);
@@ -40,7 +40,7 @@ main() {
           String tempPath = JavaFile.pathContext
               .normalize(JavaFile.pathContext.absolute(tempDir.path));
           String path = JavaFile.pathContext.join(tempPath, 'foo.dart');
-          expect(JavaFile.pathContext.isAbsolute(path), isTrue);
+          expect(JavaFile.pathContext.isAbsolute(path), isTrue, reason: '"$path" is not absolute');
           // prepare a relative path
           // We should not check that "relPath" is actually relative -
           // it may be not on Windows, if "temp" is on other disk.