gc: skip undefined symbols in import .

Fixes #1284.

R=ken3, rsc
CC=golang-dev
https://golang.org/cl/3210041

src/cmd/gc/subr.c

@@ -363,6 +363,8 @@ importdot(Pkg *opkg, Node *pack)
 		for(s = hash[h]; s != S; s = s->link) {
 			if(s->pkg != opkg)
 				continue;
+			if(s->def == N)
+				continue;
 			if(!exportname(s->name) || utfrune(s->name, 0xb7))	// 0xb7 = center dot
 				continue;
 			s1 = lookup(s->name);

test/fixedbugs/bug313.dir/a.go

+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "fmt"
+
+func a() {
+	fmt.DoesNotExist() // ERROR "undefined"
+}

test/fixedbugs/bug313.dir/b.go

+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import . "fmt"
+
+func b() {
+	Println()
+}

test/fixedbugs/bug313.go

+// errchk $G -e $D/$F.dir/[ab].go
+
+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 1284
+
+package bug313
+
+/*
+6g bug313.dir/[ab].go
+
+Before:
+bug313.dir/b.go:7: internal compiler error: fault
+
+Now:
+bug313.dir/a.go:10: undefined: fmt.DoesNotExist
+*/