Add support for labels

asm.js

@@ -112,6 +112,7 @@ const Action = Object.freeze({
     DATA:   11,
     ALIGN:  12,
     DEF:    13,
+    LABEL:  14,
 });
 
 const types = {
@@ -224,6 +225,8 @@ class Parser {
             this.state = state;
             return;
         }
+        if (token.endsWith(":"))
+            return { type: Action.LABEL, name: token.slice(0, -1) };
         const code = this.translate_code(token);
         if (code)
             return { type: Action.APPEND, code };
@@ -630,6 +633,7 @@ export class Assembler {
             [Action.DATA]:   (action) => this.action_data(action),
             [Action.ALIGN]:  (action) => this.action_align(action),
             [Action.DEF]:    (action) => this.action_def(action),
+            [Action.LABEL]:  (action) => this.action_label(action),
         };
 
         this.exports = [];
@@ -741,6 +745,10 @@ export class Assembler {
         this.defs[action.def.name] = action.def.value;
     }
 
+    action_label(action) {
+        this.defs[action.name] = this.pos.addr;
+    }
+
 	push(chunk) {
 		const text = this.decoder.decode(chunk, { stream: true });
 		for (const action of this.parser.handle(text))