exp/eval: Always read float literals as base 10

We were letting bignum decide, which caused problems with float literals with a leading 0.

R=gri
CC=golang-dev
https://golang.org/cl/816047

src/pkg/exp/eval/expr.go

@@ -762,7 +762,7 @@ func (a *exprInfo) compileCharLit(lit string) *expr {
 }
 
 func (a *exprInfo) compileFloatLit(lit string) *expr {
-	f, _, n := bignum.RatFromString(lit, 0)
+	f, _, n := bignum.RatFromString(lit, 10)
 	if n != len(lit) {
 		log.Crashf("malformed float literal %s at %v passed parser", lit, a.pos)
 	}

src/pkg/exp/eval/expr_test.go

@@ -142,6 +142,7 @@ var exprTests = []test{
 
 	Val("+1", bignum.Int(+1)),
 	Val("+1.0", bignum.Rat(1, 1)),
+	Val("01.5", bignum.Rat(15, 10)),
 	CErr("+\"x\"", opTypes),
 
 	Val("-42", bignum.Int(-42)),