Don't try to call read, if len i 0.

runtime/bin/socket.cc

Index: runtime/bin/socket.cc
diff --git a/runtime/bin/socket.cc b/runtime/bin/socket.cc
index fca6c54e253e47e33e465e818c68081f065d4a7e..fc528393b14b39fc7e746a2abce0084c065a8cd8 100644
--- a/runtime/bin/socket.cc
+++ b/runtime/bin/socket.cc
@@ -111,19 +111,17 @@ void FUNCTION_NAME(Socket_Read)(Dart_NativeArguments args) {
     intptr_t bytes_read = Socket::Read(socket, buffer, length);
     if (bytes_read == length) {
       Dart_SetReturnValue(args, result);
-    } else if (bytes_read < length) {
+    } else if (bytes_read > 0) {
+      uint8_t* new_buffer = NULL;
+      Dart_Handle new_result = IOBuffer::Allocate(bytes_read, &new_buffer);
+      if (Dart_IsError(new_result)) Dart_PropagateError(new_result);
+      ASSERT(new_buffer != NULL);
+      memmove(new_buffer, buffer, bytes_read);
+      Dart_SetReturnValue(args, new_result);
+    } else if (bytes_read == 0) {
       // On MacOS when reading from a tty Ctrl-D will result in reading one
       // less byte then reported as available.
-      if (bytes_read == 0) {
-        Dart_SetReturnValue(args, Dart_Null());
-      } else {
-        uint8_t* new_buffer = NULL;
-        Dart_Handle new_result = IOBuffer::Allocate(bytes_read, &new_buffer);
-        if (Dart_IsError(new_result)) Dart_PropagateError(new_result);
-        ASSERT(new_buffer != NULL);
-        memmove(new_buffer, buffer, bytes_read);
-        Dart_SetReturnValue(args, new_result);
-      }
+      Dart_SetReturnValue(args, Dart_Null());
     } else {
       ASSERT(bytes_read == -1);
       Dart_SetReturnValue(args, DartUtils::NewDartOSError());