Generate sh-compatible binstubs on Unix.

Previously, binstubs used [[ ]], which is a bash-specific construct.

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BUG=21854

Review URL: https://codereview.chromium.org//800733003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@42308 260f80e4-7a28-3924-810f-c04153c831b5

Author: nex3

sdk/lib/_internal/pub/lib/src/global_packages.dart

@@ -694,7 +694,7 @@ $invocation "\$@"
 # The VM exits with code 255 if the snapshot version is out-of-date.
 # If it is, we need to delete it and run "pub global" manually.
 exit_code=\$?
-if [[ \$exit_code != 255 ]]; then
+if [ \$exit_code != 255 ]; then
   exit \$exit_code
 fi
 

sdk/lib/_internal/pub_generated/lib/src/global_packages.dart

@@ -975,7 +975,7 @@ $invocation "\$@"
 # The VM exits with code 255 if the snapshot version is out-of-date.
 # If it is, we need to delete it and run "pub global" manually.
 exit_code=\$?
-if [[ \$exit_code != 255 ]]; then
+if [ \$exit_code != 255 ]; then
   exit \$exit_code
 fi