proto: allow text proto map keys to be omitted or order swapped
diff --git a/proto/text_parser.go b/proto/text_parser.go
index 8fa9f3a..0b8c59f 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -602,8 +602,9 @@
 
 			// The map entry should be this sequence of tokens:
 			//	< key : KEY value : VALUE >
-			// Technically the "key" and "value" could come in any order,
-			// but in practice they won't.
+			// However, implementations may omit key or value, and technically
+			// we should support them in any order.  See b/28924776 for a time
+			// this went wrong.
 
 			tok := p.next()
 			var terminator string
@@ -615,32 +616,39 @@
 			default:
 				return p.errorf("expected '{' or '<', found %q", tok.value)
 			}
-			if err := p.consumeToken("key"); err != nil {
-				return err
-			}
-			if err := p.consumeToken(":"); err != nil {
-				return err
-			}
-			if err := p.readAny(key, props.mkeyprop); err != nil {
-				return err
-			}
-			if err := p.consumeOptionalSeparator(); err != nil {
-				return err
-			}
-			if err := p.consumeToken("value"); err != nil {
-				return err
-			}
-			if err := p.checkForColon(props.mvalprop, dst.Type().Elem()); err != nil {
-				return err
-			}
-			if err := p.readAny(val, props.mvalprop); err != nil {
-				return err
-			}
-			if err := p.consumeOptionalSeparator(); err != nil {
-				return err
-			}
-			if err := p.consumeToken(terminator); err != nil {
-				return err
+			for {
+				tok := p.next()
+				if tok.err != nil {
+					return tok.err
+				}
+				if tok.value == terminator {
+					break
+				}
+				switch tok.value {
+				case "key":
+					if err := p.consumeToken(":"); err != nil {
+						return err
+					}
+					if err := p.readAny(key, props.mkeyprop); err != nil {
+						return err
+					}
+					if err := p.consumeOptionalSeparator(); err != nil {
+						return err
+					}
+				case "value":
+					if err := p.checkForColon(props.mvalprop, dst.Type().Elem()); err != nil {
+						return err
+					}
+					if err := p.readAny(val, props.mvalprop); err != nil {
+						return err
+					}
+					if err := p.consumeOptionalSeparator(); err != nil {
+						return err
+					}
+				default:
+					p.back()
+					return p.errorf(`expected "key", "value", or %q, found %q`, terminator, tok.value)
+				}
 			}
 
 			dst.SetMapIndex(key, val)