go/parser: imaginary constants and ! may start an expression

Complete list of tokens that can start a top-level expression.

R=rsc
CC=golang-dev
https://golang.org/cl/5671074

src/pkg/go/parser/parser.go

@@ -1861,10 +1861,10 @@ func (p *parser) parseStmt() (s ast.Stmt) {
  case token.CONST, token.TYPE, token.VAR:
  s = &ast.DeclStmt{Decl: p.parseDecl()}
  case
- // tokens that may start a top-level expression
- token.IDENT, token.INT, token.FLOAT, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operand
- token.LBRACK, token.STRUCT, // composite type
- token.MUL, token.AND, token.ARROW, token.ADD, token.SUB, token.XOR: // unary operators
+ // tokens that may start an expression
+ token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
+ token.LBRACK, token.STRUCT, // composite types
+ token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
  s, _ = p.parseSimpleStmt(labelOk)
  // because of the required look-ahead, labeled statements are
  // parsed by parseSimpleStmt - don't expect a semicolon after