Add some type inference for "yield" statements.

pkg/front_end/testcases/inference/block_bodied_lambdas_async_star.dart.strong.expect

+library test;
+import self as self;
+import "dart:async" as asy;
+import "dart:core" as core;
+
+static method main() → dynamic {
+  () → asy::Stream<core::num> f = () → asy::Stream<core::num> /* originally async* */ {
+    dynamic :controller;
+    dynamic :async_op_then;
+    dynamic :async_op_error;
+    dynamic :await_jump_var = 0;
+    dynamic :await_ctx_var;
+    dynamic :saved_try_context_var0;
+    dynamic :saved_try_context_var1;
+    function :async_op([dynamic :result, dynamic :exception, dynamic :stack_trace]) → dynamic yielding 
+      try
+        try {
+          #L1:
+          {
+            if(:controller.add(1))
+              return null;
+            else
+              [yield] null;
+            asy::Stream<core::double> s;
+            if(:controller.addStream(s))
+              return null;
+            else
+              [yield] null;
+          }
+          return;
+        }
+        on dynamic catch(dynamic :exception, dynamic :stack_trace) {
+          :controller.addError(:exception, :stack_trace);
+        }
+      finally {
+        :controller.close();
+      }
+    :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+    :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+    :controller = new asy::_AsyncStarStreamController::•(:async_op);
+    return :controller.stream;
+  };
+  asy::Stream<core::num> g = f.call();
+  asy::Stream<core::int> h = f.call();
+}