Throw and a small fix to If.

frog/leg/ssa/builder.dart

Index: frog/leg/ssa/builder.dart
diff --git a/frog/leg/ssa/builder.dart b/frog/leg/ssa/builder.dart
index 8699cad1f92c35b219f60a9ce573272eb809f034..d7b3bfa58a509dee865f01c09c5f50ad78be450d 100644
--- a/frog/leg/ssa/builder.dart
+++ b/frog/leg/ssa/builder.dart
@@ -323,7 +323,13 @@ class SsaBuilder implements Visitor {
   }
 
   visitThrow(Throw node) {
-    compiler.unimplemented("SsaBuilder.visitThrow");
+    if (node.expression === null) {
+      compiler.unimplemented("SsaBuilder: throw without expression");
+    }
+    visit(node.expression);
+    add(new HThrow(pop()));
+    // A throw aborts the building of the current block.
+    block = null;
   }
 
   visitTypeAnnotation(TypeAnnotation node) {