[tools] Fix heapsnapshot console crashes

`Console.cursorPosition` can return null when you are typing too
fast because it is implemented by sending an escape code to
the terminal and reading what it responds. This response is
naturally intermingled with normal input and the implementation
does nothing to guard against that.

We do not really need to know the cursor position though
we are simply interested in keeping cursor on the same row.
But this can be achieved by sending _horizontal position absolute_
which only changes column rather than _cursor position_
which sets both row and column.

[email protected]

TEST=manually tested

Change-Id: Ie1a064e37b90bc4529ac4f5c1259642ad5680ca6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388340
Commit-Queue: Slava Egorov <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

.gitignore

@@ -112,3 +112,4 @@ logs/logs.json
 logs/results.json
 .dart_tool/bisect_dart/
 doc/api/
+runtime/tools/heapsnapshot/.dart_tool

runtime/tools/heapsnapshot/lib/src/console.dart

@@ -21,12 +21,13 @@ class SmartConsole extends Console {
     ];
   }
 
+  void moveCursorToColumn(int column) {
+    write('\x1b[${column + 1}`');
+  }
+
   ReadLineResult smartReadLine() {
     final buffer = LineEditBuffer();
 
-    int promptRow = cursorPosition!.row;
-
-    cursorPosition = Coordinate(promptRow, 0);
     drawPrompt(buffer);
 
     while (true) {
@@ -67,16 +68,14 @@ class SmartConsole extends Console {
         }
       }
 
-      cursorPosition = Coordinate(promptRow, 0);
       drawPrompt(buffer);
     }
   }
 
   void drawPrompt(LineEditBuffer buffer) {
     const prefix = '(hsa) ';
 
-    final row = cursorPosition!.row;
-
+    moveCursorToColumn(0);
     setForegroundColor(ConsoleColor.brightBlue);
     write(prefix);
     resetColorAttributes();
@@ -93,7 +92,7 @@ class SmartConsole extends Console {
       write(buffer.text);
     }
 
-    cursorPosition = Coordinate(row, prefix.length + buffer.index);
+    moveCursorToColumn(prefix.length + buffer.index);
   }
 }