cmd/compile/internal/big: update and apply vendor.bash

Package-external tests must use the vendored math/big package, not
the original one, otherwise tests may fail if there are discrepancies
in the implementation.

Change-Id: Ic5f0489aa6420ffea1f488633453f871ce1f0f66
Reviewed-on: https://go-review.googlesource.com/10380Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/big/example_test.go

@@ -5,9 +5,9 @@
 package big_test
 
 import (
+	"cmd/compile/internal/big"
 	"fmt"
 	"log"
-	"math/big"
 )
 
 func ExampleRat_SetString() {

src/cmd/compile/internal/big/floatexample_test.go

@@ -5,9 +5,9 @@
 package big_test
 
 import (
+	"cmd/compile/internal/big"
 	"fmt"
 	"math"
-	"math/big"
 )
 
 func ExampleFloat_Add() {

src/cmd/compile/internal/big/vendor.bash

@@ -15,9 +15,15 @@ rm *.go
 cp $BIGDIR/*.go .
 
 # Use pure Go arith ops w/o build tag.
-sed 's/^\/\/ \+build math_big_pure_go$//' arith_decl_pure.go > arith_decl.go
+sed 's|^// \+build math_big_pure_go$||' arith_decl_pure.go > arith_decl.go
 rm arith_decl_pure.go
 
+# Import vendored math/big in external tests (e.g., floatexample_test.go).
+for f in *_test.go; do
+	sed 's|"math/big"|"cmd/compile/internal/big"|' $f > foo.go
+	mv foo.go $f
+done
+
 # gofmt to clean up after sed
 gofmt -w .