Add a function to parse metadata into a usable object.

lib/src/runner/parse_metadata.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.
+
+library unittest.runner.parse_metadata;
+
+import 'dart:io';
+
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
+
+import '../backend/metadata.dart';
+import '../util/dart.dart';
+
+/// Parse the test metadata for the test file at [path].
+///
+/// Throws an [AnalysisError] if parsing fails or a [FormatException] if the
+/// test annotations are incorrect.
+Metadata parseMetadata(String path) {
+  var testOn;
+  for (var annotation in parseAnnotations(path)) {
+    // The annotation syntax is ambiguous between named constructors and
+    // prefixed annotations. We don't want to have to parse the imports in order
+    // to resolve this ambiguity, so we just assume that unittest annotations

[Bob Nystrom, 2015/03/12 21:03:21]

Given that you are parsing the directives now, this doesn't seem like a problem.

I worry that silently dropping an annotation on the floor if it has a prefix
will cause problems for users.

[nweiz, 2015/03/12 23:40:22]

Done.

+    // will never be prefixed.
+    //
+    // The analyzer parses "@x.y()" as prefix "x", annotation "y", and named
+    // constructor null. It parses "@x.y.z()" as prefix "x", annotation "y", and
+    // named constructor "z".
+
+    // Always skip annotations with constructor names, because they must also
+    // have prefixes and we assume unittest metadata never has a prefix.
+    if (annotation.constructorName != null) continue;
+
+    var name;
+    var constructorName;
+    var identifier = annotation.name;
+    if (identifier is PrefixedIdentifier) {
+      name = identifier.prefix.name;
+      constructorName = identifier.identifier.name;
+    } else {
+      name = identifier.name;
+    }
+
+    if (name != 'TestOn') continue;
+    if (constructorName != null) {
+      throw new SourceSpanFormatException(
+          'TestOn doesn\'t have a constructor named "$constructorName".',
+          _spanFor(identifier.identifier, path));
+    }
+
+    if (annotation.arguments == null) {
+      throw new SourceSpanFormatException(
+          'TestOn takes one argument.', _spanFor(annotation, path));
+    }
+
+    var args = annotation.arguments.arguments;
+    if (args.isEmpty) {
+      throw new SourceSpanFormatException(
+          'TestOn takes one argument.', _spanFor(annotation.arguments, path));
+    }
+
+    if (args.first is NamedExpression) {
+      throw new SourceSpanFormatException(
+          "TestOn doesn't take named parameters.", _spanFor(args.first, path));
+    }
+
+    if (args.length > 1) {
+      throw new SourceSpanFormatException(
+          "TestOn takes only one argument.",
+          _spanFor(annotation.arguments, path));
+    }
+
+    if (args.first is! StringLiteral) {
+      throw new SourceSpanFormatException(
+          "TestOn takes a String.", _spanFor(args.first, path));

[Bob Nystrom, 2015/03/12 21:03:21]

Since you don't handle adjacent strings (which is fine), it might be good to be
explicit here like "single String literal".

[nweiz, 2015/03/12 23:40:22]

StringLiteral is the superclass of both adjacent strings and ccontinuous
strings.

+    }
+
+    if (testOn != null) {
+      throw new SourceSpanFormatException(
+          "Only a single TestOn annotation may be used for a given test file.",
+          _spanFor(annotation, path));
+    }
+
+    testOn = args.first.stringValue;
+  }
+
+  return new Metadata(testOn);
+}
+
+/// Creates a [SourceSpan] for [node].
+SourceSpan _spanFor(AstNode node, String path) =>
+    // Load a SourceFile from scratch here since we're only ever going to emit
+    // one error per file anyway.
+    new SourceFile(new File(path).readAsStringSync(), url: p.toUri(path))
+        .span(node.offset, node.end);