Fix try catch crasher bug due to null else case.

test/codegen/expect/try_catch.js

 var try_catch;
 (function(exports) {
   'use strict';
   // Function foo: () → dynamic
   function foo() {
     try {
       throw "hi there";
     } catch (e$) {
       if (dart.is(e$, core.String)) {
         let e = e$;
         let t = dart.stackTrace(e);
       } else {
         let e = e$;
         let t = dart.stackTrace(e);
         throw e;
       }
     }
 
   }
   // Function bar: () → dynamic
   function bar() {
     try {
       throw "hi there";
     } catch (e$) {
       let e = e$;
       let t = dart.stackTrace(e);
     }
 
   }
+  // Function baz: () → dynamic
+  function baz() {
+    try {
+      throw "finally only";
+    } finally {
+      return true;
+    }
+  }
+  // Function qux: () → dynamic
+  function qux() {
+    try {
+      throw "on only";
+    } catch (e) {
+      if (dart.is(e, core.String)) {
+        let t = dart.stackTrace(e);
+        throw e;
+      }
+    }
+
+  }
   // Function main: () → dynamic
   function main() {
     foo();
     bar();
+    baz();
+    qux();
   }
   // Exports:
   exports.foo = foo;
   exports.bar = bar;
+  exports.baz = baz;
+  exports.qux = qux;
   exports.main = main;
 })(try_catch || (try_catch = {}));