Fixes to enable building dart io implementation in mojo tree

runtime/platform/assert.h

Index: runtime/platform/assert.h
diff --git a/runtime/platform/assert.h b/runtime/platform/assert.h
index 54eb6526b34ffce74f33451cd3e098e34e0c9171..a60d85d5fa4d1879ae23f03b80fd7b0badb523c2 100644
--- a/runtime/platform/assert.h
+++ b/runtime/platform/assert.h
@@ -297,12 +297,14 @@ struct CompileAssert {
 };
 // Macro to concatenate two tokens. The helper is need to proper expansion
 // in case an argument is a macro itself.
+#if !defined(COMPILE_ASSERT)
 #define COMPILE_ASSERT_JOIN(a, b) COMPILE_ASSERT_JOIN_HELPER(a, b)
 #define COMPILE_ASSERT_JOIN_HELPER(a, b) a##b
 #define COMPILE_ASSERT(expr)                                                   \
   DART_UNUSED typedef CompileAssert<(static_cast<bool>(expr))>                 \
   COMPILE_ASSERT_JOIN(CompileAssertTypeDef, __LINE__)[static_cast<bool>(expr)  \
   ? 1 : -1]
+#endif  // !defined(COMPILE_ASSERT)
 
 #if defined(TESTING)