cmd/gc: insert \n at end of every input file

Not just the first one.

Fixes #5433.
Fixes #5913.

R=ken2
CC=golang-dev
https://golang.org/cl/12028049

src/cmd/gc/go.h

@@ -751,6 +751,7 @@ struct	Io
 	int32	ilineno;
 	int	nlsemi;
 	int	eofnl;
+	int	last;
 	int	peekc;
 	int	peekc1;	// second peekc for ...
 	char*	cp;	// used for content when bin==nil

src/cmd/gc/lex.c

@@ -329,6 +329,8 @@ main(int argc, char *argv[])
 		curio.peekc = 0;
 		curio.peekc1 = 0;
 		curio.nlsemi = 0;
+		curio.eofnl = 0;
+		curio.last = 0;
 
 		// Skip initial BOM if present.
 		if(Bgetrune(curio.bin) != BOM)
@@ -1602,7 +1604,7 @@ check:
 		}
 	case EOF:
 		// insert \n at EOF
-		if(curio.eofnl)
+		if(curio.eofnl || curio.last == '\n')
 			return EOF;
 		curio.eofnl = 1;
 		c = '\n';
@@ -1611,6 +1613,7 @@ check:
 			lexlineno++;
 		break;
 	}
+	curio.last = c;
 	return c;
 }
 

test/fixedbugs/bug435.go

@@ -12,4 +12,4 @@
 package main
 
 func foo() {
-	bar(1, // ERROR "unexpected|missing|undefined"
+	bar(1, // ERROR "unexpected|missing|undefined"
\ No newline at end of file