Add experimental option to cache the .length access in downlevel for-of emit.

src/compiler/commandLineParser.ts

@@ -146,6 +146,12 @@ module ts {
             description: Diagnostics.Preserve_new_lines_when_emitting_code,
             experimental: true
         },
+        {
+            name: "cacheDownlevelForOfLength",
+            type: "boolean",
+            description: "Cache length access when downlevel emitting for-of statements",
+            experimental: true,
+        },
         {
             name: "target",
             shortName: "t",