[dart mode] Support nested block comments

Closes codemirror#3878

Author: marijnh

mode/dart/dart.js

@@ -72,6 +72,12 @@
           return null;
         }
         return false;
+      },
+
+      "/": function(stream, state) {
+        if (!stream.eat("*")) return false
+        state.tokenize = tokenNestedComment(1)
+        return state.tokenize(stream, state)
       }
     }
   });
@@ -121,6 +127,27 @@
     return "variable";
   }
 
+  function tokenNestedComment(depth) {
+    return function (stream, state) {
+      var ch
+      while (ch = stream.next()) {
+        if (ch == "*" && stream.eat("/")) {
+          if (depth == 1) {
+            state.tokenize = null
+            break
+          } else {
+            state.tokenize = tokenNestedComment(depth - 1)
+            return state.tokenize(stream, state)
+          }
+        } else if (ch == "/" && stream.eat("*")) {
+          state.tokenize = tokenNestedComment(depth + 1)
+          return state.tokenize(stream, state)
+        }
+      }
+      return "comment"
+    }
+  }
+
   CodeMirror.registerHelper("hintWords", "application/dart", keywords.concat(atoms).concat(builtins));
 
   // This is needed to make loading through meta.js work.