https://www.dartlang.org/tools/pub/versioning
'^':支持小版本，大版本不包括，The string ^2.3.5 means “the range of all versions from 2.3.5 to 3.0.0, not including 3.0.0.”
'+':表示与上一版有变化，但API接口一致，The interpretation of each number is just shifted down one slot: going from 0.1.2 to 0.2.0 indicates a breaking change, going to 0.1.3 indicates a new feature, and going to 0.1.2+1 indicates a change that doesn’t affect the public API
如果直接用<或>约束,须加上单引号‘’，使用^,可不加引号；If the > character is in the version constraint, be sure to quote the constraint string, so the character isn’t interpreted as YAML syntax. For example, never use >=1.2.3 <2.0.0; instead, use '>=1.2.3 <2.0.0' or ^1.2.3.
https://www.dartlang.org/tools/pub/dependencies
Semantic Versioning: https://semver.org/spec/v2.0.0-rc.1.html