Bug 785094: Fix negative colspans showing up in the wrong places. r=jorendorff

js/src/frontend/BytecodeEmitter.cpp

@@ -4804,6 +4804,7 @@ EmitNormalFor(JSContext *cx, BytecodeEmitter *bce, ParseNode *pn, ptrdiff_t top)
             if (NewSrcNote2(cx, bce, SRC_SETLINE, lineno) < 0)
                 return false;
             bce->current->currentLine = (unsigned) lineno;
+            bce->current->lastColumn = 0;
         }
     }
 
@@ -5262,6 +5263,7 @@ EmitStatement(JSContext *cx, BytecodeEmitter *bce, ParseNode *pn)
     } else if (!pn->isDirectivePrologueMember()) {
         /* Don't complain about directive prologue members; just don't emit their code. */
         bce->current->currentLine = pn2->pn_pos.begin.lineno;
+        bce->current->lastColumn = 0;
         if (!bce->reportStrictWarning(pn2, JSMSG_USELESS_EXPR))
             return false;
     }

js/src/jit-test/tests/basic/bug785094.js

@@ -0,0 +1,2 @@
+// |jit-test| dump-bytecode
+Function("for(let b; true; x) \nfalse");