go/parser: better error message for declaration error

Fixes #4616.

R=rsc
CC=golang-dev
https://golang.org/cl/7069049

src/pkg/go/parser/parser.go

@@ -149,7 +149,7 @@ func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
 				}
 			}
 		} else {
-			p.errorExpected(x.Pos(), "identifier")
+			p.errorExpected(x.Pos(), "identifier on left side of :=")
 		}
 	}
 	if n == 0 && p.mode&DeclarationErrors != 0 {

src/pkg/go/parser/short_test.go

@@ -70,6 +70,7 @@ var invalids = []string{
 	`package p; func f() { select { case _ <- chan /* ERROR "expected expression" */ int: } };`,
 	`package p; func f() { _ = (<-<- /* ERROR "expected 'chan'" */ chan int)(nil) };`,
 	`package p; func f() { _ = (<-chan<-chan<-chan<-chan<-chan<- /* ERROR "expected channel type" */ int)(nil) };`,
+	`package p; func f() { var t []int; t /* ERROR "expected identifier on left side of :=" */ [0] := 0 };`,
 }
 
 func TestInvalid(t *testing.T) {