Track void context.

test/kernel/regression/for_loop.dart.txt

+library;
+import self as self;
+import "dart:core" as core;
+
+static method main() → dynamic {
+  core::int c = new core::DateTime::now().millisecondsSinceEpoch;
+  for(core::int i = 0; i.<(100); i = i.+(1)) {
+    core::print(let final #t1 = i in let final #t2 = i = #t1.+(1) in #t1);
+  }
+  for(core::int i = 0; i.<(100); c.<(42) ? throw "fisk" : i = i.+(1)) {
+    core::print(let final #t3 = i in let final #t4 = i = #t3.+(1) in #t3);
+  }
+}