Make Analyzer, VM and dart2js accept URI strings as part-of library identifier.

pkg/compiler/lib/src/parser/parser.dart

Index: pkg/compiler/lib/src/parser/parser.dart
diff --git a/pkg/compiler/lib/src/parser/parser.dart b/pkg/compiler/lib/src/parser/parser.dart
index 0ef9cb8373d292e82d3f55ef70d7bd8d86addb3b..65d84ef75350b6d8bf125427ab6f0b2b296f7a64 100644
--- a/pkg/compiler/lib/src/parser/parser.dart
+++ b/pkg/compiler/lib/src/parser/parser.dart
@@ -321,7 +321,12 @@ class Parser {
     assert(optional('part', token));
     assert(optional('of', token.next));
     Token partKeyword = token;
-    token = parseQualified(token.next.next);
+    token = token.next.next;
+    if (token.isIdentifier()) {
+      token = parseQualified(token);
+    } else {
+      token = parseLiteralStringOrRecoverExpression(token);
+    }
     Token semicolon = token;
     token = expect(';', token);
     listener.endPartOf(partKeyword, semicolon);