Support factory methods.

frog/leg/lib/core.dart

Index: frog/leg/lib/core.dart
===================================================================
--- frog/leg/lib/core.dart	(revision 2753)
+++ frog/leg/lib/core.dart	(working copy)
@@ -242,12 +242,22 @@
   throw "Unimplemented user-defined length.";
 }
 
+bool isInt(var v) {
+  return JS(@"($0 | 0) === $1", v, v);

[ahe, 2011/12/22 14:18:05]

Why not:

JS(@"($0 | 0) === $0", v);

[floitsch, 2011/12/22 14:36:19]

I asked him to do it this way. This way generating v at use-site would still be
legal for arguments to JS. If 'v' is an expression with side-effects it would
not be marked since there are two uses.
We have another guard against this, but I would actually prefer to get rid of
the other guard, since it always disallows generate-at-use-site for JS.

[ahe, 2011/12/22 14:41:22]

I'm not sure I understand. Are you saying that you requested this one:

a: JS(@"($0 | 0) === $0", v);

or this one:

b: JS(@"($0 | 0) === $1", v, v);

[floitsch, 2011/12/22 15:13:21]

sorry for the confusion. I asked him to do b. I find it cleaner if the compiler
knows how often the variable is used.

+}
+
 class int {}
 class double {}
 class String {}
 class bool {}
 class Object {}
-class List<T> {}
+class List<T> {
+  factory List(int n) {
+    if (!isInt(n)) throw "Invalid argument";
+    if (n < 0) throw "Negative size";
+    return JS(@"new Array($0)", n);
+  }
+}
 
 class Expect {
   static void equals(var expected, var actual) {